This is a continuation-in-part application of Ser. No. 295,829, filed Jan. 11, 1989, abandoned.
1. Field of the Invention
This invention relates generally to methods and apparatus for transforming thermal energy from a heat source into mechanical and then electrical form using a working fluid that is expanded and regenerated. This invention further relates to a method and apparatus for improving the thermal efficiency of a thermodynamic cycle via the heating of a multicomponent liquid working stream with heat released by the partial condensation of an expanded spent stream.
2. Brief Description of the Background Art
Methods for converting the thermal energy, that geothermal fluid releases, into electric power presents an important and growing area of energy generation. Geothermal power plants generally belong to one of two categories: namely, steam plants and binary plants
In steam plants, the geothermal source is utilized directly to produce steam. That steam is then expanded in a turbine, producing power. In binary plants, heat extracted from the geofluid is used to evaporate a working fluid that circulates within the power cycle. That working fluid is then expanded in a turbine, producing power.
One problem resulting from the use of a geothermal source is that geofluid (brine) can generally be cooled to moderate temperatures only. The reason for this is believed to be that further cooling can cause precipitation of silica, which may plug heat exchanger surfaces. Typically, geothermal brine may not be cooled to a temperature less than 160.degree.-180.degree. F. Once it reaches that temperature, it should be rejected into the geothermal strata.
The most advanced technology currently being used to convert the heat from geothermal heat sources into electric power appears to be the so-called supercritical organic Rankine cycle. That process, however, is associated with significant losses, which appear to result for the following reasons working fluid, after being condensed at ambient temperature, has to be heated by a geothermal brine which has a relatively high temperature, for the reasons stated above. As a result of such a mismatch between the temperature of the working fluid and the relatively high temperature of the geothermal brine, thermodynamic losses are incurred, leading to relatively low efficiency.
For example, the advanced geothermal power plant that is located at Heber, Calif., appears to have a thermal efficiency of about 13.2%. In contrast, the theoretical limit for the border condition at Heber is apparently about 27.15%. Thus, what appears to be the most advanced geothermal plant has a thermodynamic, or Second Law, efficiency of apparently about 48.62%. The efficiency of the subcritical organic Rankine cycle, which is widely used for geothermal application, is, of course, even lower than the efficiency of the supercritical organic Rankine cycle being used at Heber.
Replacing conventional systems, which use the thermal energy of geothermal fluids for relatively low temperature processes, with a system that more adequately matches the temperature of the working fluid with the temperature of the geothermal source may substantially reduce thermodynamic losses resulting from the temperature mismatching of conventional systems. Reducing those losses can substantially increase the efficiency of the system.